The lead author of the study, Brian Rundle said the study came about from a personal theory he had that psychopaths yawn less, due to the belief that yawns are related to empathy - which psychopaths lack.

They tested 135 students to see where they fell on the psychopath spectrum, showing them videos of other people yawning, laughing and just sitting there to see how often their response was a yawn.

They found that people less likely to yawn had less empathy - but that didn't exactly make them psychopaths...

"We know that we're not very likely to yawn in response to a stranger we don't have empathetic connections with," Rundle said.